1. Field of the Invention
This invention relates to a wavelength division multiplexing (WDM) coupler and more particularly to a temperature sensitive loss element for the WDM coupler.
2. Description of the Related Art
A WDM coupler couples light from one or more light sources onto two or more wave guides. At its simplest form, the WDM coupler has one input port and two output ports. Light input into the one input port may be split so as to exit the two output ports.
The splitting of the light is typically measured by a splitting ratio. The splitting ratio defines a percentage power of the light output at each of the two output ports of the WDM coupler as compared to a total power of the light input into the WDM coupler. A 50/50 splitting ratio, for example, means that the power exiting each of the two output ports is 50% of the power that inputs the one input port. The splitting ratio is a function of a wavelength of the light input into the WDM coupler. If the wavelength of the light input into the input port of the WDM coupler is λc, then the WDM coupler splits the light so that the power output at the two output ports is balanced, i.e., 50% of the power that is input into the WDM coupler is output at each output port. If the wavelength of the light input into the WDM coupler varies from λc, then the power output at the two output ports will not be balanced, e.g., the power at one output port is different from the power at the other output port.
The WDM coupler is temperature sensitive. The wavelength, λc, at which the WDM coupler has the 50/50 splitting ratio varies with the temperature of the WDM coupler. The temperature sensitivity of the WDM coupler is typically minimized by stabilizing the temperature of the WDM coupler. A temperature control device, for example, a temperature chamber or a heater, can maintain a constant temperature of the WDM coupler. By maintaining the constant temperature, the wavelength, λc, at which the WDM coupler has a 50/50 splitting ratio remains constant. As a result, light which is input into the WDM coupler and which has a wavelength equal to the wavelength λc for the constant temperature can be assuredly split at the splitting ratio, regardless of external temperature variations.